This invention relates to apparatus for photoacoustic (PA) imaging of structures in an optically absorbing subject and methods and apparatus for calibrating an imaging apparatus of this type. These types of imaging apparatus and methods can be useful for medical, scientific, and industrial imaging, imaging of physiological parameters and tracking of molecular probes.
Photoacoustic (PA) imaging is a method to visualize distribution of optically absorbing objects in a volume of interest. The method employs short laser pulses that illuminate the volume and cause the absorbing objects to heat up slightly and undergo thermoelastic expansion which results in outward-propagating pressure waves. By measuring the time-of-flight and profile of these pressure waves at points outside the volume, and then applying back-projection algorithms, the location and relative strengths of the photoacoustic sources can be deduced. Many approaches have been suggested for three-dimensional PA imaging. Each employs a different combination of detection scheme and image reconstruction algorithm. These can be divided into scanning methods, where a single detector is scanned along the detection surface in two dimensions and staring methods, where a 2D array of detectors is used and no scanning is necessary. A combined scanning-staring approach has been suggested as well, where a linear array of detectors is scanned in one dimension. Several methods for 3-D PA image reconstruction have been proposed as well, including the spherical Radon transform, synthetic aperture beam forming, plane-wave approximation, iterative back-projection and universal closed-form radial back-projection. It has also been shown that detector(s) line of sight toward the PA source defined the source boundaries that can be sharply reconstructed. In other words, the wider the range of viewing angles subtended by the detector(s) toward the imaging volume, the better defined the reconstructed images would be.
U.S. Pat. No. 5,713,356 to Kruger teaches methods and apparatus for measuring and characterizing the localized electromagnetic wave absorption properties of biologic tissues in vivo using incident electromagnetic waves to produce resultant acoustic waves. Multiple acoustic transducers are acoustically coupled to the surface of the tissue for measuring acoustic waves produced in the tissue when the tissue is exposed to a pulse of electromagnetic radiation. The multiple transducer signals are then combined to produce an image of the absorptivity of the tissue, which image may be used for medical diagnostic purposes. In specific embodiments, the transducers are moved to collect data from multiple locations, to facilitate imaging. In a subsequent patent to Kruger, namely U.S. Pat. No. 6,104,942, Kruger indicates that in the method described in U.S. Pat. No. 5,713,356, a large number of pulses of electromagnetic radiation (e.g. 100-100,000) spaced at a repetition interval, are generated to stimulate the tissue.
U.S. Pat. No. 5,840,023 to Oraevsky entitled “Optoacoustic Imaging for Medical Diagnosis” describes a system that utilizes time-resolved measurement of profiles of laser-induced transient pressure (acoustic) waves. These waves are emitted by acoustic sources preferentially generated in absorbing tissues of diagnostic interest. The technique allows visualization of absorbed light distribution in turbid, layered and heterogeneous tissues irradiated by laser pulses in vivo. The laser opto acoustic tomography can be used for the characterization of structure and properties of normal tissue, and for the detection of tissue pathological changes. Three-dimensional images of organs and portions of organs can be obtained.
The photoacoustic imaging method and apparatus described herein can be useful for medical imaging of non-planar light absorbing structures, such as blood vessels, tumors, and internal organs, for imaging of physiological parameters such as oxygen saturation, and for tracking of molecular probes, all in three-dimensions and at high frame rates relative to previously used methods. Embodiments of the systems described herein are suitable for small animal imaging and for clinical use, either as a stand-alone transportable device or integrated with another imaging modality, such as X-ray CT scanner, PET scanner, MRI scanner or a combination thereof. One embodiment of the imaging apparatus can acquire 3-D images with single laser shots at a frame rate of 10 Hz.
The imaging method of the present disclosure is based on the photoacoustic (PA) effect, where a short laser pulse diffusely irradiates a volume of tissue, and is absorbed in the optically absorbing structures therein. These structures undergo a slight but rapid temperature increase, resulting in elastic expansion and generation of an outgoing transient pressure wave. These pressure waves, also called photoacoustic waves, can be detected on the surface of the volume using a wide band ultrasound detector(s). From the surface PA measurements an image of the distribution and relative strength of the PA sources inside the volume can be generated. In general, the larger the solid angle of the surface of detection, the better the reconstruction of the PA sources can be because more viewing angles are captured.
According to one embodiment, the apparatus of the present disclosure uses backward mode illumination where the laser radiation impinges on the volume from the same side as the ultrasound detectors. The detectors are arranged in a sparse array that can be planar or bowl shaped or can be arranged over other surfaces. In an exemplary embodiment of the PA imaging apparatus, the array has a window in the middle for delivery of the laser beam. The subject to be imaged is placed in the area above the optical window and is fully accessible for administration of drugs or anesthetics, monitoring of vitals, or any other necessary handling. Illumination can also be accomplished with multiple laser beams from a multitude of positions and angles around the volume.